Chapter
9:
Main Gear Fairings

(Main Gear
Fairings are not addressed
in the plans.)

The
Problems

Our gear bows
are made in an airfoil shape that should help to reduce drag. But
because we install the gear bow at a canted angle to the fuselage, a
large majority of each gear leg ends up being set at a positive incidence angle relative to the oncoming airflow.
This creates drag. The drag gets worse when we add the brake conduits and
extend the trailing edges. Look at your gear bow carefully and you'll
see that most of trailing edge is not in trail with the leading edge.
If you look at the picture of my gear bow, you can clearly see that the
trailing edge is much closer to the square on the right than it is to being
mid-way between the two. (The mid-point is about where that shadow
line is on the mounting tab.) To help you visualize it, the gear is 2
inches thick at this point. The trailing edge is at 1.625 inches.
It should be at 1 inch. It's over a half an inch off from being in
trail with the leading edge! And that's after purposely offsetting it
to what I thought was higher in the previous steps! Those of you who
made your trailing edge in line with the rest of the bow will be even
farther off from parallel. Your trailing edge would probably be in
line with the square on the right! What a drag, man!

The other
problem is
that the chord line is not long enough. The speed gurus say that the
chord line for an airfoil should be about 4
times its thickness. So the goal is
to stack foam blocks onto the gear so we can reshape the legs into an
airfoil with a chord line 4
times longer than the section is thick, and to position the trailing edge
in trail with the leading edge so that the incidence angle is parallel to
the oncoming airflow. It's been said
that a "perfectly faired main gear" can gain you about 7-10 knots
of airspeed. I intend to gain that with the approach outlined below.
Mucho thanx to Ron Springer who inspired this particular
approach. The fairings are easier to build if you can take the gear out of
the plane.

The
Process

Step 1 -- At
cruise speeds, our planes fly about 2 degrees nose up. So, jack the plane off the ground so
that there’s no weight on the main wheels. Raise the nose wheel up
until the top longerons are 2 degrees nose up. Then measure the distance of the gear’s leading edge from vertical.
I made my measurement at the point where the leading edge goes into the
fuselage. For me, that distance was 3.625 inches. Record
this measurement and remove the gear from the fuselage.

Step 2 --
Set your gear bow on your table top, leading edge down. Next, make two
standoffs that are as tall as that measured in Step 1. Place these
under the gear bow at your measurement spots. Again, you do this to
approximate the gear's geometry relative to the fuselage.
I just happened to have two bricks that were exactly the right height.
So I used those. Now, measure the thickness of the gear bow in intervals
along each gear leg. This is most easily done by butting two carpenter
squares against the gear and measuring the width between the two squares.
I
measured the thickness every 3 inches. Write them down because the 4:1
ratios of these thicknesses will determine how high to stack the foam.

Step 3 -- Now
it's time to start piling on the foam. But first things first -- PLUG
UP your brake conduits on both ends! Use bolts or golf tees, but use
something. You don't want pour foam seeping inside and clogging up the
conduits! It's actually easier to pile on the foam with the gear
sitting flat on the table. So I removed the bricks and set the gear
flush on the table, leading edge down. I attached some of my
sticky-back sanding paper to the gear leg. I cut out a urethane foam
block, then rubbed the block side to side over the sand paper. This
quickly carves the block to the exact shape of the gear leg. After
carving two adjacent blocks, I removed the sticky back sand paper, vacuumed
up the dust, and laid the blocks side by side, sanded sides up onto wax
paper. (The wax paper just catches the pour foam run-offs and helps to
keep from pour foaming the top of the table. I also placed wax paper
under the gear legs for the same reason.) I mixed up a few ounces of
pour foam, then VERY QUICKLY poured the mixture over the blocks, allowing a
small portion to dribble into the crack between the two blocks. I VERY
QUICKLY took a mixing stick and VERY QUICKLY spread the mixture until the
faces of the blocks were covered. I VERY QUICKLY picked up
the blocks and positioned them into their proper places on the gear leg.
I then pushed a stack of bricks against the blocks to hold them firmly
against the gear leg until the pour foam solidified. This process took
about 10-15 minutes for each 2-block set. With blocks fitted to
the outside and inside of each leg, I trimmed the blocks flush with the
trailing edge. For me, it was just easier to glue more blocks onto a
flat trailing edge than it was to shape really tall blocks to include the
new trailing edge. The first picture is of my gear leg before
cutting the foam flush with the trailing edge. The second picture
shows the trailing edge blocks glued on (and was taken out of order after completing Step
4, sorry). Now, some people build
dams and just pour-foam the entire leg. That's certainly a quick way
of doing it. But I loathe pour foam. While quick, pour foam
expands and contracts, expands and contracts, expands and
contracts.....FOREVER. :-) I didn't want to deal with those issues.
I did use pour foam, but only as the "glue" to attach the blocks to the gear legs and to each other.
Don’t use micro because (a) it takes too long to wait for cure; and (b) the
cured micro will be harder to shape!

Step 4: -- Make
a 90-degree sanding block. I made my sanding block from two pine
boards with bracing to hold them perfectly at 90 degrees. Again, raise
the bow to achieve the distance measured in Step 1. To keep the bow
from moving, secure the bow to the table by screwing in a block of wood at
the end of each gear leg. Screw the leg to the block by using a screw
through one or two of the axle holes. Now, sand along each
side of the bow, removing the foam with each pass until you hit glass. What this does is quickly sets the
thickness of the urethane foam blocks to the same thickness of the bow.
You MUST do this sanding with the bow raised up to replicate the cant angle
to the fuselage (per step 1), or it will not work! As
you can see in the pictures, the outside of the
bow is sanded with some 40-grit sticky-backed sanding paper stuck onto the
flat board. I have just started sanding the outside of the gear bow in
the first picture. Now, this configuration won't work for the inside
of the bow. Since it is concaved, the edges of the board will grab and
gouge the foam. So I attached a large diameter PVC tube to the sanding
block. I wrapped the 40 grit sand paper around the tube. In the
second picture, you can see how I sanded the inside curve until I "hit the
glass". Pretty cool! It only took 30 minutes to sand both sides
of one gear leg! I could have done it in 20 minutes, but I was being
careful. :-) (Note: The pictures were taken before adding the
trailing edge blocks. Sorry. I got impatient and just had to try out the
sanding block! :-) )

Step 5 -- Now it's time to trim off the trailing edge. Using the
thicknesses measured in Step 2, I multiplied them by 4 (4:1 ratio,
remember?) and make two templates. I used some plastic sheeting I had
lying around. You want something really flexible because you will be
bending it around the gear leg. I measured off the 3-inch spacing
along the length, then measured up by the 4:1 ratio. When you do this,
you'll notice that the marks for the trailing edge will not line up.
This is because the gear bows are thicker in some areas than others.
The thicknesses are not uniform over the length of the legs either.
So, I took a straight edge and made a "best fit." Note that I
added a small curve at the fatter part of the template, something
approaching a 4.5:1 ratio. I'd like you to think I did that just to
make the trailing edge look snarky, but I really did it in anticipation of
fitting an intersection fairing later on between the gear legs and the
fuselage. Anyway, clamp the two templates onto the gear leg and sand
across them to trim the trailing edge to shape.

Step 6 -- Get
two 90-degree squares. Now, find the new “centerline” by measuring the
width, dividing in half, and marking the spot. Do this about every inch along
the length of the bow.

Connect the
dots! You’ll have a nice line that is the location of the new trailing
edge. Note how your new trailing edge is at a zero incidence angle compared
to the angle of the bow's cross section.

Step 7 -- Make
yourself an airfoil-shaped sanding block. I had planned to follow Todd
Parker's recommendation and use a NACA 16-021 pressure recovery section.
Problem is the leading edge portion of the gear's airfoil shape is thinner
than the 16-021 profile. I didn't want to mess with adding
micro, contouring to shape, THEN glassing. So I just decided to shape
an airfoil around the shape of the gear bow. I pulled out the
M-drawings and shaped the airfoil you see below. I made two templates,
then made the contouring sanding block you see here I applied sticky backed sandpaper
onto that portion of the sanding block that touches foam. I applied
duct tape to that portion that touches glass. The duct tape allows the
sanding block to slide along and keeps the bare glass from abrading the foam
sanding block. All I did was hold it in place and
sand the foam side to side until I reached the center line. You'll
note I have a strip of duct tape at the trailing edge. Once I got to
the centerline, I didn't want to sand past it! The duct tape prevents
the sandpaper from hitting the foam at that spot. So it prevented me
from sanding the trailing edge as I continued to shape and finesse
the rest of the foam. After I got the foam contoured, I applied
a strip of peel ply onto the foam along the new trailing edge to provide for
glass-to-glass bond later. I made a paper template and laid up two
2-UNI layups on the workbench, then transferred the layups onto the gear
bow. The 2-UNI layups were on a 30-degree
bias like we did on the wings. After cure, I trimmed the trailing edge flush with the
foam. I contoured the inside foam and puledl up the peel ply strip
from the trailing edge.
I applied flox into the trailing edge, slurried the exposed foam, then
glassed the inside of the legs with 2-UNI. After cure, I'll trim the
trailing edge and the main gear fairings will be completed! I'll then reinstall
the gear into the fuselage.

Step 8 -- Once
the gear is reset into the fuselage, I'll make some intersection
fairing between the gear bow and the fuselage. This is easily done by
stacking a small block of foam, carving to a pleasing shape that the
oncoming air will like, and glassing BID over it. You can also make the
shape from modeling clay and dig it out after.

Pictures of the
final product are on the very bottom of the next
page.